Thermomechanical insight into the reconfiguration of Diels-Alder networks

Relating thermoreversible bond kinetics to temperature and mechanical stress is essential to for the ongoing development of meltprocessable, reconfigurable networks. Here, we apply the dynamic mechanical analysis methods to study the kinetics and equilibrium behavior of dynamic polymer networks above their gel point. Thermoreversible Diel-Alder (DA) adducts are installed as linking groups to create well-defined poly(caprolactone) networks. Stress relaxation studies at various strains are performed to differentiate how temperature and stress influence the rate of bond breaking, i.e., the rate of the retro-DA reaction. The resulting thermal activation energies of stress relaxation are nearly independent of applied stress over the experimental range studied. The forward, more sluggish, DA reaction is studied by continuously monitoring the response in Young's modulus (E') following different temperature reductions. Equilibrium values of E' are used to establish the temperature dependence of the DA equilibrium constant, and a van't Hoff analysis from thermomechanical studies alone allows estimates of enthalpy and entropy changes associated with the DA reaction. The results provide insight on how to design, formulate, and reprocess thermoreversible shape memory networks. (C) 2017 The Society of Rheology.